This invention relates generally to the rendering of latent electrostatic visible using single and multiple colors of dry toner and, more particularly, to a developer apparatus including structure for ensuring and restoring the development of the edges and reducing the solid area image noise (due to carrier development in two component development systems) of the previously developed images.
Development of electrostatic images is currently available through the use of a wide variety of development systems, such as, monochrome development systems which use a single color toner and no carrier; two component development systems that employ both toner and carrier materials; and three component systems that make images visible with toner, carrier and an additive.
An example of two component development is described in the concept of tri-level xerography in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein, the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negatively and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it a cross the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development system is biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In tri-level xerography, the xerographic contrast on the charge retentive surface or photoreceptor is divided three, rather than two, ways as is the case in conventional xerography. The photoreceptor is charged, typically to 900 v. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged area development, i.e. CAD) stays at the full photoreceptor potential. The other image is exposed to discharge the photoreceptor to its residual potential, (typically 100 v) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD).
Various techniques have heretofore been employed to develop electrostatic images as illustrated by the following disclosures which may be relevant to certain aspects of the present invention and are incorporated herein by reference, as well as the references cited in them, to the extent necessary to practice the present invention.
As disclosed in U.S. Pat. No. 3,457,900, magnetic brushes have been designed to give fringe field or solid area development by adjusting the conductivity of the carrier. It is also stated therein that they can also be made to tone areas of less charge and clean areas of greater charge giving what is known in the art as a reverse development.
U.S. Pat. No. 4,847,655 discloses a highlight color imaging apparatus which includes magnetic brush development system that has a plurality of developer housings each including a plurality of magnetic brush rolls associated therewith. Conductive magnetic brush (CMB) developer is provided in each of the developer housings. The CMB developer is used to develop electronically formed images. The physical properties such as conductivity, toner concentration and toner charge level of the CMB developers are such that density fine lines are satisfactorily developed notwithstanding the presence of relatively high cleaning fields.
But even with these and other developer techniques, it has been found that two component development of the second color image in tri-level highlight color (lines and solids) often produces ragged edges of images and high background toning. The first developed image of a tri-level copy (or print) is degraded by the second development process. This degradation is most severe when the toner concentration of the first developer increases and the triboelectricity decreases. This degradation is noticed the most on the lead edge of lines that are perpendicular to the process direction. The lead edges of the perpendicular lines are ragged and have a higher level of background toner due to toner particles sprayed from the lines or solids.
The present invention aims at improving image edge smoothness, solid area granularity, and background cleanliness in two component development copier/printers by providing a process and apparatus whereby electrical and/or mechanical forces (such as ultrasonic or AC biased electrodes) are supplied so that the toner can rearrange itself according to the electric fields from the image.